Method of and apparatus for measuring small changes in length



Nov. 14, 1939. J. SIVERTSEN METHOD OF AND APPARATUS FORMEASURING SMALL CHANGES IN LENGTH Filed May 28, 1957 3 Sheets-Sheet 1 INVENTOR rwwfahM ATTORN EY Nov. 14, 1939. J. SIVERTSEN 2,180,175

METHOD OF AND APPARATUS FOR MEASURING SMALL CHANGES IN LENGTH I Filed May 28, 1937 3 Sheets-Sheet 2 I I w 1;

INVENTOR JW 8? v ATTORNEY J. SIVERTSEN Nov. 14, 1939.

HEIHOD OF AND APPARATUS FOR MEASURING SMALL CHANGES IN LENGTH Filed May 28, 1937 3 Sheets-Sheet 3 INVENTOR P 6 M Mf M ATTORNEY Patented Nov. 14,1939

METHOD OF AND APPARATUS FOR MEAS- URING SMALL CHANGES IN LENGTH Jens Sivertsen, Philadelphia, Pa., assignorf to: Ti'nius Olsen Testing Machine Company, Phil-- adelphia, Pa., a corporation of Pennsylvania Application May 28, 1937, Serial No. 145,19

4 Claims. (01. 177-351) In a copending application, I have described and broadly claimed a novel method of and apparatus for measuring small changes in length and particularly the changes of length taking place in a specimen under stress in a testing machine. A change in length of the specimen under test caused a change in configuration of electrical parts to cause an electrical voltage change in a circuit which was amplified to drive an electric specimen. A second unit, carefully matched to be electrically equal to the first unit is located at a different place in the equipment and'electrically balanced against the first unit, and this second unit is readjusted to restore the balance'to the 5 electrical circuit. To state it more simply, a change in distance causes an electrical change in the electric circuit which causes avoltage, and this voltage will, by electromotive means, cause motor to cause a readjustment of said configuraanother voltage equal to the first voltage. 10

tion where the change occurs. The accuracy of the system is therefore de In my present invention, I eliminate any mependent on the degree to which the two electrical chanical connection between the motor driven units having air gaps are equal. v

parts and the parts that connect with the speci- With the above and other objects in view as men. The instrument should operate and also will hereinafter clearly appear, my invention 15 record length with approximately the same relacomprehends a novelmethod of and apparatus tive accuracy, i. e. in per cent of the elongation. for measuring small .changes in length. The apparatus is therefore easier to operate, Other novel stepsof the method and features cheaper to manufacture, and a high degree of acof construction will be set forth in the detailed curacy is obtained. It has fewer mechanical parts description and the appended claims. 20 to be driven by the motor, thereby enabling one For the purpose of illustrating the invention, to use a smaller motor. The accuracy we are I have shown in the accompanying drawings aiming at, can easily be found as follows: If the typical embodiments of it, which in practice, will apparatus is used to determine the modulus of give satisfactory and reliable results. It is, howelasticity of steel having a proportional limit of ever, to be understood that the various instru- 25 30,000 lbs. per square inch, the total elongation mentalities of which my invention consistscan up to this limit is approximately .001 per inch be variously arranged and organized, and myinspecimen. In a standard specimen of 2" gauge vention is not limited to the exact arrangement length we therefore get an elongation of .002". and organization of these instrumentalities as If one desires an accuracy of one tenth of one herein set forth. 30 per cent, or 5 of 33 the necessary accuracy Figure l is a diagrammatic view illustrative of is %000000 or 2.10 inches, and if the accuracy exthe mechanical parts and the electrical circuits, pected is one-half per cent, the necessary 2.0- for carrying out in practice my novel method. curacy is 5.10". Figure 2 is a top plan view of the instrument.

35 If the apparatus is to have the versatility of Figure 3 is a side elevation of the instrument. 35 a commercial product, it must also be able to Figure 4 is a side elevation partly broken away measure much greater distances, possibly not so of an electric unit and indicating or recording accurately considering the use. mechanism.

After the elastic limit has been passed, many Figure 5 is an end elevation of Figure 4.

40 specimens will elongate considerably with a small Figure 6 is a detail of gearing seen in Figures! 40 increase in load, in some instances as much as and 5. ten per cent or more. A ten per cent elongation Figure 7 is a side elevation on an enlarged scale is equal to 0.2. If the specimen breaks, it should and partly broken away, of a portion of Figure 4. not cause breaking of the instrument. Figure 8 is an elevation of a modified coil and These requirements are very difficult to aclever arrangement which can be substituted for 45 complish if a balanced mechanical configuration that seen in Figure 1. has to be maintained in a small instrument at- Figure 9 is an elevation of a modified form of tached to the specimen and with a drive shaft the other coil and lever arrangement seen in cooperating to effect it mechanically, as it is dif- Figure l. r V v I 5U ficult to keep the alignment of the shaft and other Figure 10 is an enlarged detail .of the laminaarrangements working properly without a comtions of the coil 6, showing the position the parts plicated structure. assume when the upper laminations are fixed to In accordance with the present invention, an the lever and in center position. electrostatic or an electromagnetic unit has an Figure 11 is a view similar to Figure 10 but air gap affected by a change in length of the showing the relative position of thelaminations 55 The coil '6 is supplied through resistor 4 and a conductor 8, and a conductor 9 leads from the coil 6 to a point I0 which is one side of an impedance II forming part of a control I2. This .control may be of any desired or conventional character. It may or may not have thermionic valves, or it may have relays. The impedance II may be the primary of an input transformer, or the coil of a relay. The coil 1 obtains its electrical energy from plug 3 through variable resistors 5, conductors I3 and return conductor I4 leading to the point II] of the control I2.

The resistor 4 and resistor 5 are used for the following reasons:

If the resistance part of the impedance in the coil 6 including the losses in the iron core I6 and I1 and the resistance in the conductors 8 and 9 is not equal to that of the opposing coll I including the losses in the core I8 and I9 of coil 6 and the resistance in conductors I3 and I4, then a balance can be obtained by the adjust able resistors 4 and 5.

The cores I6iI and I8--I9 are laminated.

The total resistance from point 20 through coil 6 to point I0 can thus be made equal to the resistance between point 2i through coil 1 and point I0. By adjustment of the air gap in coil 1, we can make the inductance of coil I equal to the inductance of coil 6, since the two resistors I and 2 are very closely matched.

We do not have any voltage between points I5 and I0, and no current in the impedance II of the control I2, when the adjustments of resisters 4 and 5, and the air gaps in coll "I, has been completed as outlined.

The control I2 is electrically connected to a motor 22 to control its direction of rotation. The current supply for the motor may be either A. C. or D. C. and is led from its source of supply through a plug 23.

The control I2 may also control the speed of the motor in any conventional manner well .known in the art of motor control, depending on the voltage between points It! and I5.

The shaft of the motor 22 drives directly or indirectly by a gear 24, a gear 25 on the shaft of a drum 26 to drive the drum or pointer 27.

The motor 24 also drives a gear 28 on a micrometer screw 29. This drive may be direct or through intermediate gears.

The rotation of the micrometer screw, the drum and the pointer are thus synchronized.

One end of the micrometer screw bears against a ball 30 inserted in and end of a bell crank lever 3| fulcrumed at 32 to a fixed point. The opposite end of the lever 3| has the lamination I9 pivoted or fixed to it. The fixation or lamination I9 to lever 3|, and lamination ill to lever 39 can be accomplished in two ways. One way is shown in Figures 8 and 9. The other way is as shown in Figures 1, 2, 3, 4 and 5. The relative merits of these two arrangements will be explained. The laminations I8 and I9 together 33 designates the specimen unler test, which is being stressed in a testing machine. An extensometer 34 is clamped on the specimen by the knife edge 35 and spring 36 are rigidly connected to the frame of the extensometer when the specimen is clamped in position. The specimen is also clamped between the spring 38 and the knife edge 3'! on a tool steel lever 39 which at its opposite end bears against a tool steel pivot 40 which forms a part of the rigid frame of the extensometer 34. The laminations IT in Figures 1, 2 and 3 are pivotally connected with the lever 39, and, with the laminations I6 and coil 6, form an inductance substantially equal to that of the laminations I8 and I9 and coil 1.

When the specimen .33 is stressed, the distance between the knife edges will vary, and, assuming that an elongation takes place, the lever 39 will move on its pivot and the laminations I1 will make a proportional outward movement, thereby opening up the air gap to create an unbalance the electrical network. This causes the control I2 to regulate the motor 22 to cause it to revolve in a direction to drive the micrometer screw 29 to actuate the lever 3| to open the air gap between laminations I8 and I9, and when the opening between laminations I8 and I9 is the same as between I6 and H, the inductances are equal, and the electrical unbalance caused by laminations I1 and coil 6 have been matched by the unbalance in coil 1, due to the movement of the laminations I9.

To express it more correctly, the two movements are equal when the electrical unbalance caused by the movement of laminations I1 is matched by that caused by the movement of the laminations I9.

The amount of rotation by the micrometer screw is one measurement, and the rotation of the indicating pointer 21 or the drum 2S, indicates or records the amount of elongation of the specimen.

The system, as shown in very simple. The difficulties are to make the two air gaps, the one in the extensonieter and the one remote therefrom, open up in exactly the same manner. If

:this is not accomplished, the errors are so great that the apparatus will be without merit.

The arrangement with a lever to obtain, in stead of a straight lined movement of the laminations, a rotating movement of the lever and the laminations around the seat in the pivot member 49, and around-the pivot 32, the construction shown in Figures 8 and 9 will'give good results. However, if this arrangement is used, the distances a and a and b and b, must be respectively made equal, as shown in Figures 8 and 9.

In Figure 10 the laminations are shown in starting pdsition, while in Figure 1] they are shown as they appear after the air gap has opened. The opening is unequal as the upper laminations take a certain angle compared to their former position. By making a equal a, and b equal b, we get very good results, and the limits of the obtainable accuracy using this method are the smoothness of the laminations.

A better method is shown in Figures 1 to 6. The upper laminations are here shown as being hung on a pivot point on the lever. By true rotating movements we thereby obtain a straight and the pivot member 40.

line opening of the air gaps. The lever 39 is inserted in a pointed, hardened piece of drill rod 4|. to the piece 42. A screw 43 is threaded into the part 42 and has a seat for the other end of the piece H. The axis of 42 is located on a line between points 46 and 41. 46 is a contact point between the lever 39 and the specimen 33, and 41 is the contact point between the lever 39 The piece 42 and laminations I1 are free to rotate around the point 45. When the lever 39 including the axis 45, rotates around the point 41, and 42 rotates around point 45, we have two rotating movements. For small movement of the point 46, we will therefore have a straight lined opening of the air gaps. Substantially the same arrangement is employed for the laminations I9 to obtain a straight lined opening of the air gap. Under these conditions, the two laminations do not need to have the same distance from their respective pivotal points as they both open in a straight line direction.

It is important that the movement of the lever 39 takes place in substantially a single plane and for this reason I provide means to guide the free end of the lever, and resilient means to cause such free end to be retained against a polished guide.

As shown in Figure 12, the spring 35 is slotted at 48, so that a wall of the spring forms a guide for the free end of the lever which is highly polished to reduce friction. A spring 49 has one end connected to a side of the lever and its other end connected to the frame, thereby tending to retain the free end of. the lever against the guide. The pivot forming members 50 are adjustable for proper positioning of the fulcrum of the lever. When the extensometer is clamped to a specimen to be tested, it is locked in the usual manner by a locking lever 5|.

The trunnions forming the pivot 32 carry a weight 52, see Figures 4 and 5, the purpose of which is to keep the laminations l9 hanging vertically as the lever 3| rotates on its fulcrum 32.

It is to be understood that where in the claims I refer to a specimen, it is not with the limited interpretation of a part taken from a whole for the purpose of testing, since it is intended to include not only a part selected or cut out for a specimen but as well any object, body or mass oi material of which a test is to be made.

Having thus described by invention, what'I claim as new and desire to secure by Letters Patent is:

1. An extensometer for measuring changes in length of a specimen under stress, comprising a frame, means on the frame to clamp the specimen in one section, means on the frame including a spring and lever to clamp the specimen in another section a predetermined distance from said first section, said lever being fulcrumed at one end and having a contact point at the opposite end, and a magnetic unit having a lamihated core pivotally supported by said lever at A flat spring 44 with a seat 45 is fastened a point between its contact point and fulcrum and having a cooperating laminated magnetic core rigidly fixed to the frame of said extensnmeter and said magnetic unit changing its electrical characteristics due to movements of said lever.

2. An extensometer for measuring changes in length of a specimen under stress. comprising a frame, means on the frame to clamp the specimen in one section. means on the frame includ ing a spring and lever to clamp the specimen in another section a predetermined distance from said first section, said lever being fulcrumed at one end and having a contact point at the opposite end. a magnetic unit having a. laminated core pivotally supported by said lever at a point between its contact point and fulcrum and having a cooperating laminated magnetic core rigidly fixed to the frame of said extensometer and said magnetic unit changing its electrical characteristics due to movements of said lever and means to prevent lateral movement of said lever during its turning movement.

3. An extensometer for measuring changes in length of a specimen under stress, comprising a. frame, means on the frame to clamp the specimen in one section, means on the frame including a spring and lever to clamp the specimen in another section a predetermined distance from said first section, said lever being fulcrumed at one end and having a contact point at the opposite end, a magnetic unit having a laminated core pivotally supported by said lever at a point between its contact point and fulcrum and having a cooperating laminated magnetic core rigidly fixed to the frame of said extensometer and said magnetic unit changing its electrical characteristics due to movements of said lever, means to prevent lateral movement of said lever during its turning movement. a guide fixed to said frame, and resilient means tending to move the free end of said lever towards said guide.

4. An extensometer for measuring changes in length of a specimen under stress, comprising a frame, means on the frame to clamp the specimen in one section, means on the frame including a spring and lever to clamp the specimen in another section a predetermined distance from said first section, said lever being fulcrumed at one end and having a contact point at the opposite end, a magnetic unit having a laminated core pivotally supported by said lever at a point between its contact point and fulcrum and having a cooperating laminated magnetic core rigidly lever.

JENS SIVERTSEN. 

